1. Field of the Invention
This invention relates generally to wireless data communications, and more specifically to a physical layer for an ultra wide band (UWB) system that utilizes the unlicensed 3.1 GHz-10.6 GHz UWB band, as regulated in the United States by the Code of Federal Regulations, Title 47, Section 15.
2. Description of the Prior Art
An important parameter in the design of a UWB system is choice of the operating bandwidth. This choice impacts not only the link budget and correspondingly the overall system performance, but also affects the receiver, especially in terms of the LNA and mixer design, and the speed at which the digital-to-analog converters (DACs), the analog-to-digital converters (ADCs), and ultimately the baseband signal processing operate.
The overall system performance is related to the received power, which is a function of the difference between the total transmit power and the path loss. Since the FCC defines average power in terms of dBm per MHz, the total transmit power can be expressed completely in terms of the operating bandwidth. If the lower frequency fL of the operating bandwidth is fixed at 3.1 GHz and upper frequency fU is varied between 4.8 GHz and 10.6 GHz, then the total transmit power PTX(fU) can be expressed as follows:PTX(fU)=−41.25+10 log10(fU−fL)(dBm)This equation assumes that the transmit power spectral density is flat over the entire bandwidth. The path loss, which attenuates the transmitted signal, is also a function of the lower and upper frequencies of the operating bandwidth. The path loss model specified by the IEEE 802.15.3a channel modeling committee is given as follows:
                    P        L            ⁡              (                              f            g                    ,          d                )              =          20      ⁢                        log          10                ⁡                  [                                    4              ⁢                                                          ⁢              π              ⁢                                                          ⁢                              f                g                            ⁢              d                        c                    ]                      ,where fg is defined as the geometric average of the lower and upper frequencies, d is the distance measured in meters, and c is the speed of light.
The effects of increasing the upper frequency past 4.8 GHz are described herein below. In FIG. 1, the received power 10 at a distance of 10 meters as a function of the upper frequency is plotted. From this figure, it can be seen that the received power increases by at most 2.0 dB (3.0 dB) when the upper frequency is increased to 7.0 GHz (10.5 GHz). On the other hand, increasing the upper frequency to 7.0 GHz (10.5 GHz) results in the noise figure for the broadband LNA increasing by at least 1.0 dB (2.0 dB). All relative changes in received power and noise figure were made with respect to an upper frequency of 4.8 GHz. Thus, the overall link margin will increase by at most 1.0 dB when increasing the upper frequency past 4.8 GHz, but at the expense of higher complexity and higher power consumption.
Another important criterion to keep in mind when selecting the operating bandwidth is that interferers may potentially lie within the band of interest. For example, in the United States, the U-NII band occupies the bandwidth from 5.15 GHz-5.85 GHz, while in Japan, the U-NII band occupies the bandwidth from 4.9 GHz-5.1 GHz. Both of these U-NII bands lie right in the middle of the allocated UWB spectrum 20 (see FIG. 2). If a UWB device uses an upper frequency that is larger than 6.0 GHz, then it will have to deal with the interference produced by IEEE 802.11a systems. It may be possible to mitigate, to some extent, this interference by using either static or adaptive notch filters or by using complicated baseband mitigation algorithms at the UWB receiver; but such mitigation will come at the expense of increased complexity. Conversely, the same UWB device will generate interference for IEEE 802.11a systems. To prevent generation of this interference, UWB devices will have to incorporate a notch filter at the transmitter to prevent emission within the U-NII band 22. Effectively, the presence of the U-NII band 22 breaks the UWB spectrum 20 into two distinct and orthogonal bands that are free from interference: 3.1 GHz-4.8 GHz, and 6.0 GHz-10.6 GHz (see FIG. 2).
Since the gains from using the higher band (6.0 GHz-10.6 GHz) are incremental, it would be both advantageous and desirable to provide a UWB system that uses the lower band 3.1-4.8 GHz. Other reasons for using the smaller operating bandwidth include: 1) front-end RF components, such as the LNA and mixer, can be built in current CMOS technologies with low noise figure; and 2) the signal processing can be done at lower speeds, implying that the sampling rates for the ADC can be smaller, and the timing requirement can be relaxed. As a result, the final solution will have lower complexity and can be manufactured using standard, and mature CMOS technologies, which implies an early time-to-market and low cost and low power solution.
In view of the foregoing, it would be both advantageous and desirable in the wireless data communication art to provide a physical layer for an ultra wide band (UWB) system that utilizes the unlicensed 3.1 GHz-10.6 GHz UWB band, as regulated in the United States by the Code of Federal Regulations, Title 47, Section 15. The reasons for choosing the lower band include, among others:
Incremental gains from larger operating bandwidths,
Lower sampling rates for the ADC,
Relaxed timing requirements,
Complete CMOS solutions for the proposed UWB system,
Lower cost,
Lower power,
Early time-to-market, and
Scalability.